In a typical multi-engine aircraft electrical system for which the invention is particularly suited, each engine drives a generator through a constant speed drive. Each of the engine driven generators powers a set of load circuits and the load circuits may be interconnected through an electrical distribution bus. An auxiliary power unit drives an auxiliary generator to supplement the engine driven generators in the event of an engine or generator failure. The auxiliary power unit is also used to provide electric power with the aircraft on the ground when the engines are shut down and an external power source is not readily available.
A generator control unit (GCU) for each of the generators monitors the electrical condition of the associated generator and conditions of the mechanical drive to the generator. A bus power control unit (BPCU) monitors power distribution throughout the system and the condition of the generators, and controls loads and bus tie breakers which connect the electrical distribution bus between the generators, loads and the auxiliary generator. One or both loads are powered by one of the available generators. Two generators are not, however, connected in parallel. The control units have in the past coordinated interlock information using multiple hard wired circuits.